4 DISCUSSION
This study shows that plateau pika disturbance is related to lower palatable plant biomass and higher plant species richness across five sites or at an individual site, demonstrating that plateau pika disturbance decreases the forage availability service and increases the biodiversity conservation service, in agreement with the first hypothesis. Plateau pika disturbance in relation to lower palatable plant biomass has also been reported in a previous study (Liu et al., 2017), while prairie dog disturbance has been reported to be related to higher palatable plant biomass (Martínez-Estévez et al., 2013). Prairie dog disturbance encourages the arid grassland in North America to shift from annual plants to perennial plants, in which blue gramma (Bouteluoa gracilis ) and vine mesquite (Panicum obtusum ) are usually perennial plants that are palatable for livestock (Sierra-Corona et al., 2015), whereas plateau pika disturbance enables alpine grassland composition to change from sedge to unpalatable broad-leaved plants in the Qinghai-Tibetan Plateau (Pang & Guo, 2018). In this case, plateau pikas preferentially consume dicotyledons, and most dicotyledons are unpalatable plants (Pang & Guo, 2017; Zhao et al., 2013). Thus, plateau pika consumption can benefit palatable plant growth (Pang & Guo, 2017) since it can alleviate the competitive pressure of monocotyledons, and these monocotyledons are palatable plants (Li et al., 2015; Zhao et al., 2013). However, plateau pikas often bury many plants in the process of producing bare soil patches, thereby reducing aboveground plants at the plot scale. The decrease in palatable plant biomass, which is derived from bare soil patches and sedge replacement by unpalatable broad-leaved plants, is greater than the increase in palatable plant biomass derived from the alleviation effect of plateau pika consumption. Therefore, plateau pika disturbance contributes to a negative effect on palatable plant biomass, resulting in a decrease in forage availability services. In addition, plateau pika disturbance in relation to higher plant species richness is similar to that of other small burrowing herbivore disturbances, such as that of the plains pocket gopher (Rogers et al., 2001), the coexisting kangaroo rat and mole rat (Davidson & Lightfoot, 2008), and the prairie dog (Hagenah & Bennett, 2013). Higher plant species richness caused by plateau pikas can be explained by several mechanisms: first, plateau pika activities are likely to enhance the environmental heterogeneity (Guo et al., 2012b; Pang et al., 2020a; Yu et al., 2017a), which creates gaps for opportunistic plant species (Hagenah & Bennett, 2013; Pang & Guo, 2017); second, bare soil patches caused by plateau pikas provide additional good habitat for plant seeds to germinate and settle (Pang & Guo, 2017) because it is difficult for plant seedlings to survive on vegetated surfaces with 7 to 8 cm root mats with less soil and water supplies (Li et al., 2015); third, the burrow of plateau pikas is the only habitat home for snow finch (Montifringilla ruficollis ) (Liu et al., 2013), and this bird can recruit more plant seeds in areas with plateau pikas. These three processes encourage plateau pika disturbance to increase the biodiversity conservation service.
Plateau pika disturbance is associated with lower soil water storage across all five sites or at each individual site, in contrast to the second hypothesis. This result has also been found in a previous study (Liu et al., 2013), whereas it is different from prairie dog disturbance (Martínez-Estévez et al., 2013). The different responses of soil water storage to disturbance by small burrowing herbivores are ascribed to the soil porosity and compaction of grassland. Prairie dog disturbance increases soil water storage by reducing the compaction and increasing the pore spaces of arid grassland (Martínez-Estévez et al., 2013), while plateau pika disturbance has no impact on the soil bulk density of alpine grassland (Pang et al., 2020b) and encourages more topsoil water to infiltrate into deep soil though bare soil patches and burrow entrances in root mats (Li et al., 2015), contributing to a decrease in soil water storage in the topsoil layer. In addition, bare soil patches caused by plateau pikas can increase water evaporation in topsoil because of the lack of vegetation insulation. Thus, the plateau pika disturbance results in low soil water storage in the topsoil layer, contributing to a reduction in the water conservation service.
Plateau pika disturbance correlates with higher soil organic carbon stock and does not relate to soil total potassium stock across the five sites or individual site, whereas plateau pika disturbance has different impacts on soil total nitrogen stock and soil total phosphorus stock among five sites, demonstrating that there is a general pattern for plateau pika disturbance in relation to carbon sequestration and soil potassium maintenance services, and no general pattern concerns the effects of plateau pika disturbance on the soil total nitrogen and total phosphorus maintenance services. These results support the third hypothesis but do not support the fourth hypothesis. In this case, plateau pika disturbance can increase the soil organic carbon stock in two possible ways: first, plateau pika disturbance can input extra organic matter through the deposition of uneaten food (Liu et al., 2009; Liu et al., 2013; Yu, et al., 2017a; Zhang et al., 2016) and the excretion of urine and feces (James et al., 2009; Yu et al., 2017b); second, the burrowing behavior of plateau pikas can protect the soil organic matter pools from ultraviolet light and cannot be blown away by wind (Pang et al., 2020a), which reduces the decomposition, mineralization and removal of soil organic matter (Clark et al., 2016; Yurkewycz et al., 2014). Since the responses of soil total nitrogen and total phosphorus maintenance to plateau pika disturbance are site-dependent, which is similar to previous studies (Pang et al. 2020b), soil total nitrogen and phosphorus maintenance is not suitable for estimating the effect of disturbance by small burrowing herbivores on the ecosystem services of alpine grasslands.
Additionally, this study found that plateau pika disturbance intensity was also related to palatable plant biomass, plant species richness, soil water storage, soil organic carbon stock, and soil total nitrogen and phosphorus stocks. As the disturbance intensity increases, the palatable plant biomass, plant species richness, soil organic carbon stock, soil total nitrogen and phosphorus stocks show downward parabolas, demonstrating that there is a threshold of plateau pika disturbance intensity for maximizing the forage availability, biodiversity conservation, carbon sequestration, and soil nitrogen and phosphorus maintenance services. When the disturbance intensity is within the threshold of disturbance intensity, plateau pika disturbance can enhance soil total nitrogen (Li et al., 2014) and organic carbon accumulation (Yu et al., 2017b), increase palatable plant biomass (Pang & Guo, 2018) by improving the growth potential of grass plants (Wang et al., 2012b), and encourage more hygrophytes and mesophytes, annual and perennial, common and rare plants to coexist (Guo et al., 2012b), contributing to higher forage available, biodiversity conservation, carbon sequestration, soil total nitrogen and phosphorus maintenance services. When the disturbance intensity is below the threshold of disturbance intensity, the dominant sedge plants place great competitive pressure on grass plants, which leads grass plants to maintain a low percentage (Pang & Guo, 2018; Wang et al., 2012b) and makes it difficult for rare plants to coexist. Once the disturbance intensity surpasses its threshold, low soil water content in alpine grassland (Liu et al., 2013) only sustains the xerophytes and mesophytes, most of which are unpalatable (Pang & Guo, 2018), resulting in reductions in palatable plant biomass and plant species richness. Notably, the responses of plant species richness to plateau pika disturbance intensity showed linear increases at Luqu and Gonghe, which were ascribed to the disturbance intensity. The disturbance intensities at these two sites were lower than the threshold of disturbance intensity (Table 2), which validates the general patterns of plateau pika disturbance intensity in relation to plant species richness. Plateau pika disturbance can increase the input of soil organic matter (Pang & Guo, 2017; Pang et al., 2020a), and this increase in soil organic matter is lower when the disturbance intensity is below the threshold. Low vegetation biomass at high disturbance intensities (Pang & Guo, 2017; Pang et al., 2020a; Sun et al., 2015) also decreases the input of soil organic matter. Thus, soil organic carbon sequestration and soil nitrogen and phosphorus maintenance services are low when the disturbance intensity is below or over the thresholds.
In contrast to forage availability, biodiversity conservation, carbon sequestration, soil nitrogen and phosphorus maintenance services, the water conservation service shows a decreasing trend as the plateau pika disturbance intensity increases, which is ascribed to evaporation and water infiltration on bare soil patches. Previous studies have shown that evaporation and water infiltration from topsoil to deep soil are higher in bare soil patches than in vegetated surfaces (Liu et al., 2013; Yu et al., 2017b). As the soil bare area increases, the amount of water evaporation and infiltration shows an increasing trend (Liu et al., 2013), which encourages the soil water storage to decrease, contributing to a gradual decrease in the water conservation services of alpine grasslands.
Data from one site show that prairie dog disturbance has a positive impact on grassland ecosystem services in arid regions of North America (Martínez-Estévez et al., 2013), whereas data from five sites show that plateau pika disturbance and its disturbance intensity have a positive or negative impact on different ecosystem services of alpine grasslands in the Qinghai-Tibetan Plateau. Thus, a general pattern can be identified for the effect of plateau pika disturbance on forage availability, biodiversity conservation, water conservation and carbon sequestration services, whereas the soil nutrient maintenance service in relation to plateau pika disturbance varies by site. These results indicated that plateau pika control should consider the management target of ecosystem services for alpine grasslands and its disturbance intensity threshold in the Qinghai-Tibetan Plateau. The findings of this study present a possible pattern of how disturbance by small burrowing herbivores influences grassland ecosystem services and open a broader vision for insight into small burrowing herbivore in relation to grassland ecosystem services.
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